The production of chlorine is essentially carried out by electrolysis of alkali chloride solutions, in particular sodium chloride solutions, by means of three alternative technologies based on diaphragm, mercury cathode or, in the most advanced case, ion-exchange membrane electrolysers, equipped with anodes consisting of expanded or variously perforated titanium sheets provided with an electrocatalytic coating comprising platinum group metals and/or oxides thereof, optionally in admixture. Anodes of such kind are, for instance, commercialised by Industrie De Nora under the trademark DSA®. A common problem to the three technologies is the need of limiting the molar oxygen content in chlorine at levels below 2% and preferably not higher than 1% Oxygen is generated by the unavoidable secondary reaction of water oxidation and hampers most processes making use of chlorine, in particular dichloroethane synthesis, which is the first step of PVC production. According to teachings of the prior art, in order to obtain low oxygen contents the anodes, whose coating is obtained by painting the titanium substrate with a noble metal precursor solution subsequently decomposed by a thermal treatment, are then subjected to a final thermal treatment which entails some energy consumption penalties, estimated on average at about 50-100 kWh/tonne of product depending on the duration and on the temperature applied.
The same anodes are, moreover, employed in hydrochloric acid electrolysis, which is acquiring a growing interest since hydrochloric acid is the typical by-product of all major chlorine-using industrial processes. The increase in the productive capacity of present-day plants involves the generation of remarkable quantities of acid whose allocation on the market is significantly difficult. Hydrochloric acid electrolysis leads to formation of chlorine which can be recycled upstream giving rise to a substantially closed cycle, free of significant environmental impact, which is nowadays a decisive factor to obtain the construction licenses from the competent authorities. The problem characterising the application of noble metal-coated titanium anodes in this context is directly associated with the strong aggressiveness of hydrochloric acid The latter, penetrating through the defects of the electrocatalytic coating, corrodes the titanium-coating interface and provokes the detachment thereof in a relatively short time, with consequent plant shut-down.
A first countermeasure suggested by the prior art, consisting of employing substrates made of titanium-palladium alloy, which is renowned for its peculiar corrosion resistance and used for the construction of critical equipment of chemical plants, has led to no sensible result. A second remedy, consisting of improving the protection of the titanium substrate by increasing the thickness of the catalytic coating, could not be applied beyond certain limits, as it has been observed that excessively thick coatings become extremely brittle and are therefore subject to remarkable detachment phenomena of purely mechanical nature. The preferred solution so far provides the electrocatalytic coating to be obtained as a multiplicity of overlaid individual layers. The thus-obtained anode presents a reduced number of defects and is therefore characterised by a better operative lifetime. Nevertheless, it has been observed that the advantages in terms of prolonged lifetime are counterbalanced by penalties in terms of higher operative voltages, entailing an electrical energy consumption increase of about 50-150 kWh/tonne of chlorine.
Similar problems arise also in all those electrochemical processes, in particular electrometallurgical ones, wherein noble metal-coated titanium electrodes are used as oxygen-evolving anodes: these processes often involve the use of highly concentrated acidic solutions, in particular by sulphuric acid, which turn out to be aggressive for the currently employed titanium substrates. Measures such as those recalled for the hydrochloric acid case are routinely applied with the purpose of obtaining acceptable lifetimes.